It is widely known to perform a heat treatment (such as hardening, tempering, nitriding or carburization) on a steel product used as a part of an automobile, such as a gear or a shaft, in order to improve the mechanical properties thereof. Besides, a surface treatment method that performs a shot peening treatment (shot peening will be abbreviated as “SP”, hereinafter) for imparting a compressive residual stress on the steel product subjected to the heat treatment is widely adopted (see Patent Literature 1, for example).
The heat treatment is usually performed in a batch process, so that the degree of the heat treatment varies among the steel products. Thus, even if the SP treatment conditions are properly managed, the degree of the surface treatment on the steel products varies. Thus, it is required to inspect whether the surface treatment has been properly performed on the steel products or not.
In Patent Literature 2, the inventor has disclosed a surface characteristics inspection apparatus capable of inspecting the surface treatment state of a processed material, such as a steel material, subjected to a surface treatment.